1. Field of the Invention
This invention relates to a method and apparatus for baseband signal carrier recovery of a suppressed carrier modulation signal and, more particularly, to a multidetector which receives baseband inputs based on the modulated signal and generates outputs for use by multiple carrier recovery systems.
2. Discussion
In the operation of communication systems, a message signal modulates a carrier signal in order to encode the message information into the modulated signal. The modulated signal is transmitted to a receiver which demodulates the signal, separating the message signal from the carrier signal. Key operations in practical receivers are automatic gain control (AGC), carrier phase tracking, sweep-aided carrier acquisition, and lock detection.
One modulation format that is commonly used in communication systems is quadrature amplitude modulation (QAM). In the QAM format two message signals amplitude-modulate a pair of quadrature carrier signals, i.e., two sinusoids having the same frequency and a phase angle difference of 90 degrees. Examples of QAM special cases are binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 8 phase shift keying (8 PSK), and rectangular 16-state QAM (16QAM).
The modulation format of the transmitted signal is commonly designed to achieve suppression of the residual carrier. This reduces the energy required to transmit the message with the same fidelity. Carrier suppression can be achieved by symmetrically arranging the phase and amplitude relationships of QAM modulation states, and encoding the data so that each of the modulation states are statistically equally likely.
The extended Costas loop and the remodulation loop are two commonly used circuits for carrier phase tracking of suppressed carrier modulated signals. The addition of amplitude detection circuitry for use in AGC and lock detection circuitry for use in sweep-aided carrier acquisition complicates the design of the receiver, especially when these circuits are implemented in such a way that they process the modulated carrier directly.
It is an object of the present invention to provide a method of using the demodulated low frequency in-phase and quadrature-phase baseband signals to derive the amplitude detection and lock detection signals in addition to the phase error signal.
It is a further object of the present invention to provide a circuit that implements the method of using the demodulated baseband signals to derive the amplitude detection, lock detection, and phase error signals, and combines common elements to reduce the complexity of the circuit.
This invention is directed to a circuit for recovering a carrier of a suppressed carrier modulated signal having a high frequency. The circuit includes an in-phase and quadrature-phase detector, wherein the detector converts the incoming signal to a baseband signal having a frequency less than the modulated signal. A multidetector receives the baseband signal output by the detector. The multidetector operates on the lower frequency baseband signal and generates a plurality of output signals. The plurality of output signals determine an error signal which varies in accordance with a phase error between a desired carrier and a received carrier signal. A phase-lock-loop detector receives the error signal and generates a lock signal input to the detector to vary the phase of the detector.
This invention is also directed to a method of recovering a carrier of a suppressed carrier modulated signal having a high frequency. The method includes the steps of demodulating the modulated signal into a baseband signal having in-phase and quadrature-phase components, where the in-phase and quadrature-phase components have a frequency less than that of the modulated signal. The method also includes the step of generating a plurality of output signals in accordance with the baseband signal and generating an output error signal in accordance with at least one of the plurality of output signals. The output error signal defines a phase error between a desired carrier signal and a received carrier signal. The method also includes varying the output of a phase lock loop in accordance with the error signal and controlling the demodulation of the modulated signal in accordance with the output of the phase lock loop in order to lock onto the carrier signal.